Thin film magnetic recording disks and disk drives are conventionally employed for storing large amounts of data in magnetizable form. In operation, a typical contact start/stop (CSS) method commences when a data transducing head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk where it is maintained during reading and recording operations. Upon terminating operation of the disk drive, the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk and stopping.
For optimum consistency and predictability, it is necessary to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Accordingly, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head. However, if the head surface and the recording surface are too smooth, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces, eventually leading to what is referred to as a "head crash." Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
Conventional practices for addressing these apparent competing objectives involve providing a magnetic disk with a roughened recording surface to reduce the head/disk friction by techniques generally referred to as "texturing." Conventional texturing techniques involve mechanical polishing or laser texturing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the textured surface on the substrate is intended to be substantially replicated in the subsequently deposited layers. The surface of an underlayer can also be textured, and the texture substantially replicated in subsequently deposited layers.
Conventional longitudinal recording media typically comprise a substrate, such as aluminum (Al) or an Al alloy, e.g., aluminum-magnesium (Al--Mg) alloy, plated with a layer of amorphous nickel-phosphorus (NiP). Alternative substrates include glass, ceramic, glass-ceramic, and polymeric materials and graphite. The substrate typically contains sequentially deposited on each side thereof at least one underlayer, such as chromium (Cr) or a Cr-alloy, e.g., chromium vanadium (CrV), a cobalt (Co)-base alloy magnetic layer, a protective overcoat typically containing carbon, and a lubricant. The underlayer, magnetic layer and protective overcoat, are typically sputter deposited in an apparatus containing sequential deposition chambers. A conventional Al-alloy substrate is provided with a NiP plating, primarily to increase the hardness of the Al substrate, serving as a suitable surface to provide a texture, which is substantially reproduced on the disk surface.
In accordance with conventional practices, a lubricant topcoat is uniformly applied over the protective overcoat to prevent wear between the disk and head interface during drive operation. Excessive wear of the protective overcoat increases friction between the head and disk, thereby causing catastrophic drive failure. Excess lubricant at the head-disk interface causes high stiction between the head and disk. If stiction is excessive, the drive cannot start and catastrophic failure occurs. Accordingly, the lubricant thickness must be optimized for stiction and friction.
A conventional material employed for the lubricant topcoat comprises a perfluoro polyether (PFPE) which consists essentially of carbon, fluorine and oxygen atoms. The lubricant is typically dissolved in an organic solvent, applied and bonded to the carbon overcoat of the magnetic recording medium by techniques such as dipping, buffing, thermal treatment, ultraviolet (UV) irradiation and soaking. A significant factor in the performance of a lubricant topcoat is the bonded lube ratio which is the ratio of the amount of lubricant bonded directly to the carbon overcoat of the magnetic recording medium to the amount of lubricant bonded to itself or to a mobile lubricant. Desirably, the bonded lube ratio should be between 0.3 to 0.7 (e.g. about 0.5 (50%)) to realize a meaningful improvement in stiction and wear performance of the resulting magnetic recording medium.
The escalating requirements for high areal recording density impose increasingly greater requirements on thin film magnetic recording media in terms of coercivity, stiction, squareness, medium noise and narrow track recording performance. In addition, increasingly high areal recording density and large-capacity magnetic disks require smaller flying heights, i.e., the distance by which the head floats above the surface of the disk in the CSS drive (head-disk interface). For conventional media design, a decrease in the head to media spacing increases stiction and drive crash, thereby imposing an indispensable role on the carbon-protective overcoat.
There are various types of carbon, some of which have been employed for a protective overcoat in manufacturing a magnetic recording medium. Such types of carbon include hydrogenated carbon, graphitic carbon or graphite, and nitrogenated carbon or carbon nitride and hydrogen-nitrogenated carbon. These types of carbon are well known in the art and, hence, not set forth herein in great detail.
Generally, hydrogenated carbon or amorphous hydrogenated carbon has a hydrogen concentration of about 5 at. % to about 40 at. %, typically about 20 at. % to about 30 at. %. Hydrogenated carbon has a lower conductivity due to the elimination of the carbon band-gap states by hydrogen. Hydrogenated carbon also provides effective corrosion protection to an underlying magnetic layer. Amorphous carbon nitride, sometimes referred to as nitrogenated carbon, generally has a nitrogen to hydrogen concentration ratio of about 5:20 to about 30:0. Hydrogen-nitrogenated carbon generally has a hydrogen to nitrogen concentration ratio of about 30:10 to 20:10 (higher concentration of hydrogen than nitrogen). Amorphous (a) hydrogen-nitrogenated carbon can be represented by the formula a-CH.sub.x N.sub.y, wherein "x" is about 0.05 (5.0 at. %) to about 0.20 (20 at. %), such as about 0.1 (10 at. %) to about 0.2 (20 at. %), and "y" about 0.03 (3.0 at. %) to about 0.30 (30 at. %), such as about 0.03 (3.0 at. %) to about 0.07 (7.0 at. %). A particularly suitable composition is a-CH.sub.0.15 N.sub.0.05. Graphitic carbon or graphite contains substantially no hydrogen and nitrogen.
The drive for high areal recording density and, consequently, reduced flying heights, challenges the capabilities of conventional manufacturing practices. For example, a suitable protective overcoat must be capable of preventing corrosion of the underlying magnetic layer, which is an electrochemical phenomenon dependent upon factors such as environmental conditions, e.g., humidity and temperature. In addition, a suitable protective overcoat must prevent migration of ions from underlying layers into the lubricant topcoat and to the surface of the magnetic recording medium forming defects such as asperities. A protective overcoat must also exhibit the requisite surface for wear resistance, lower stiction, and some polarity to enable bonding thereto of a lubricant topcoat in an adequate thickness.
Furthermore, as the head-disk interface decreases to less than about 1 .mu.inch, it is necessary to reduce the thickness of the carbon-containing protective overcoat to below about 100 .ANG. to improve performance of the magnetic recording medium and reduce the spacing loss between the read/write head and magnetic recording medium surface. However, when the thickness of the carbon-containing protective overcoat is reduced to below about 100 .ANG., corrosion protection is adversely affected. In addition, head crash is encountered because it exhibits very poor tribological properties and low reliability. Most GMR and MR media overcoats comprise a single layer of carbon material, such as amorphous hydrogenated carbon or amorphous nitrogenated carbon and exhibit adequate reliability at a thickness of about 125 .ANG. to about 250 .ANG.. However, as the thickness of the carbon-containing overcoat is reduced to below about 100 .ANG., head crash occurs, presumably because of lower wear resistance and the discontinuities formed in the sputter deposited layer.
Prior attempts have been made to reduce the thickness and increase the density of carbon-containing protective overcoats. Such techniques include plasma-enhanced chemical vapor deposition and ion-beam deposition. However, such prior attempts have met with a disadvantageously high defect count and lack of manufacturing feasibility.
Prabhakara et al., in U.S. Pat. No. 5,855,746, discloses a magnetic recording medium having a plurality of carbon-containing protective layers with an outer nitrogen-containing layer, wherein nitrogen is excluded from the initial carbon deposition for improved coercivity. Hwang et al., in U.S. Pat. No. 5,785,825, disclose a dual phase carbon overcoat including an initial amorphous carbon film on a magnetic layer and a doped amorphous carbon film sputter deposited on the amorphous carbon film. Lal et al., in U.S. Pat. No. 5,714,044, disclose a magnetic recording medium containing first and second carbon overcoats, wherein the second carbon overcoat is deposited under a nitrogen-containing atmosphere. Onodera, in U.S. Pat. No. 5,607,783, discloses a magnetic recording medium containing single or plural carbon-containing protective layers with increasing hydrogen content. Nagao et al., in U.S. Pat. No. 4,869,797 disclose a method of sputter depositing a carbon protective layer with a bias voltage of -10V to -100V applied in the vicinity of the support and magnetic layer.
In copending U.S. Pat. application Ser. No. 09/065,014 filed on Apr. 21, 1998 a multilayer protective overcoat is disclosed which contains a first hydrogenated carbon protective overcoat and a second protective overcoat of graphitic carbon or carbon nitride. In copending U.S. patent application Ser. No. 09/161,278 filed on Sep. 28, 1998 a magnetic recording medium is disclosed containing an amorphous carbon overcoat and a nitrogenated carbon overcoat deposited thereon.
There exists a continuing need for a magnetic recording medium comprising a protective overcoat capable of satisfying the imposing demands for high areal recording density, reduced head-disk interface and corrosion protection. There also exists a particular need for a magnetic recording medium having a protective overcoat with a thickness of less than about 100 .ANG. affording superior corrosion protection, exhibiting excellent tribological properties at very low glide heights and having long term durability.